have 04.5, 2500, H.O. 600, tow package, 48re, 265/70r17 tires, stock truck. Bought used with 15000 miles, first tank hand calc. was 15.4. Only mods to date is to put drain plugs in diff. covers. Symptom: At highway speeds get a vibration and/or harmonic in drivetrain that goes away when putting it in tow/haul mode. Anyone else experience this and is it a problem or just a condition? #2: Has anyone run 235 tires and towed with them on a swd? I am a fan of narrow tires except on my tractor.